Photovoltaic cells are a major technology for generating electricity which is being deployed ever more widely. Improvements in the efficiency and cost of such cells are important.
In photovoltaic cells, light is absorbed in a semiconductor, which creates electron-hole pairs. Electrons then travel to one contact while holes travel to the opposite contact, thus producing electric current. The creation of electrons and holes via a photon-induced electronic interband transition is a necessary, but not sufficient, condition for practical electricity production. It is also in practice necessary that the optical absorption resulting from electronic excitation be the dominant absorption mechanism, otherwise the efficiency will be below acceptable levels. Free carrier absorption is where the energy of incident light is absorbed by free carriers inside the material and results in the free carriers gaining momentum. Free carrier absorption competes with optical absorption resulting in electronic transitions. Thus for solar cell designs using highly doped silicon, free carrier absorption is a limiter to solar cell efficiencies, particularly in the infrared region of light. (See reference (1) below.)
A solar cell requires an internal bias. Usually this internal bias is created by a p-n junction, which is obtained by doping the material. However, doping the material increases the concentration of free carriers and thus increases free carrier absorption and shifts this absorption to higher energies. In addition, increasing the doping increases the bulk recombination rate thus decreasing the conversion efficiency. Reasonably high doping levels are needed to keep sheet resistance low. Therefore, solar cells are often designed to have a very shallow and highly doped emitter region so that the sheet resistance is low and the free carrier absorption and the average bulk recombination rates are small, but this approach limits the thickness of the depletion region and hence how much current light can create in the device.
Materials with dimensions smaller than the free carrier diffusion length (nanomaterials) have suppressed free carrier absorption. (See, e.g., reference (2) below.) In this respect nanomaterials would be ideal for solar cells. In addition, nanomaterials have increased absorption and low reflection, which is also ideal for solar cells. However, a solar cell needs a conductive path for the free carriers to travel to the junction. In nanoparticles, a type of nanomaterial with dimensions reduced in all three directions, carriers need to hop or tunnel from one particle to the next. Since hopping and tunneling are inefficient, a highly resistive processes, nanoparticles are non-ideal for solar applications. On the other hand, nanowires, where dimensions are reduced in only two directions, retain the advantage of suppressed free carrier absorption for light with an electric field perpendicular to the wire axis, while allowing low-resistance transport parallel to the wire axis.
Recently one group placed nanowires on top of a silicon solar cell, as depicted schematically in FIG. 1. In this design, the nanowires are not in electrical contact with the bulk silicon, not doped, and not aligned. The observed efficiency gain in these structures is possibly because the nanowires act like an anti-reflection coating for the bulk cell. Since the nanowires are not vertical to the substrate and not in electrical contact with the substrate, the maximum benefit from nanowires is not realized.
There have been proposals to use nanowires with concentric n and p regions. (See references (3), (4), and (5) below.) This device design is said to have unique advantages, including that the optical absorption length is decoupled from the free carrier diffusion length. However one deficiency of this design is that the junction area is very large and therefore leakage current will need to be controlled.
Others have proposed using silicon nanowires where one section of the wire is n type and the other section of the wire is p type. (See reference (6) below.) This design also has advantages, but as with the radial design, one possible limitation is the leakage current caused by surface states that run through the depletion region and junction.
Kayes, Atwater and Lewis (reference (7) below) performed calculations to better understand both radial (FIG. 2) and planar n-p junctions (FIG. 3) for photovoltaic applications. The calculations found that the quasi-neutral regions away from the depletion zone can tolerate more traps and larger recombination rate, which may be a result of fewer minority carriers in these regions to contribute to recombination. However, the calculations show that a low trap density is desirable in the depletion zone to achieve high efficiencies. Moreover, since nanowires have a large surface area, increased traps and recombination in the depletions zone may be expected.
Other groups have proposed photovoltaic devices where one material type (or doping) is made up of nanowires, and another material type (or doping) is made up of bulk material, as schematically depicted in FIG. 4. A junction is thus formed at the interface between the nanowires and the bulk material. Like the other designs, the nanowires are in the depletion region and at the junction, again yielding a device limited by surface recombination and carrier transport. (See references (8), (9), and (10) below.)
The designs described above reap some of the benefits of nanowire solar cells, but either do not take full advantage of these benefits or gain these at the expense of increased leakage current from nanowires in the depletion region.
There is therefore a continuing need for designs of nanowire solar cells which can achieve higher efficiency and lower cost.